The present invention pertains to a polymerization procedure for producing a linear polyethylene from ethylene with a narrow molecular-weight distribution. It is understood that the term "polymerization" includes the copolymerization of ethylene with an alpha-olefin, ethylene remaining the major element for fabricating a linear polyethylene. In the conventional fabrication of linear polyethylenes, ethylene represents a minimum of 70% by weight of the total monomers involved.
The procedure in accordance with the invention is comprised of gas-phase polymerization of ethylene in the presence of a catalytic component containing at least one transition metal, magnesium and a halogen, and a co-catalyst composed of an alkylaluminum and an electron donor; the electron donor is characterized by being selected from among the monoethers.
In the polymerization of propylene, it is known to necessarily combine an electron donor with the titanium halide type co-catalyst component on a support comprising magnesium. This combination is indispensable for obtaining isotacticity and stereoregularity in the polymerization of propylene. In this case the presence of the electron donor facilitates obtainment of good orientation of the CH.sub.3 branchings of the chain.
Although the presence of the electron donor allows an improvement in the stereoregularity, it is known to be a catalyst poison in that it generally harms the productivity of the said catalyst by decreasing it very greatly. Therefore, in the case of the polymerization of propylene, it is necessary to find a compromise in the addition of electron donor: sufficient for obtaining good stereoregularity but not too much for obtaining a reasonable productivity of the catalyst. Among other places, this can be found in U.S. Pat. No. 4,511,703, one of the numerous patents relative to Ziegler catalysts of propylene polymerization in which an electron donor is taken into account. Among these patents, U.S. Pat. Nos. 4,535,069, 4,304,891, 4,387,328, 4,363,901 and 4,473,660 can be cited in which the sole focus is on the polymerization or copolymerization of propylene and the stereoregularity of the polymer.
This question of stereoregularity is not raised in the case of the polymerization of ethylene or mixtures in which it is found in excess. Therefore, the employment of an electron donor, which can only act negatively by reducing the productivity or the activity of the catalyst or, in addition, by introducing problems of odor, is only done very selectively with precise goals in combination with the co-catalyst of ethylene polymerization. Conventionally, as for example in U.S. Pat. No. 4,497,904, ethylene is polymerized in the presence of a titanium-based catalytic component and an alkylaluminum as co-catalyst, with the co-catalyst not containing an electron donor.
In European Patent application 232,643, the employment of an electron donor combined with the co-catalyst makes it possible to preserve the spherical structure of a spherical catalytic component entering in the composition of a stable, spherical catalyst of ethylene polymerization. In this application, most of the known electron donors are suitable for obtaining the desired result.
For the purpose of fabricating polyethylenes with a high apparent density with good catalytic productivity, it is recommended in French Application 2,531,717 that the co-catalyst and the titanium-based catalytic component be combined with an electron donor from the silane family. These products have the drawback of undergoing degradation reactions in the presence of alkylaluminum, as reported by E. Vahasarja et al., Journal of Polymer Science 25, 3241 (1987).
For the same purpose, it is proposed in European Patent application 155,682 to combine with the catalytic systems for ethylene polymerization an electron donor containing at least one nitrogen atom in its molecule. However, this type of electron donor has the drawback of creating odor problems and/or residual toxicity in the polymer.